cell cycle and celldivision-notes 1-20-1.pdf

 CELL CYCLE AND CELL DIVISION 1. Introduction Topics Discussed The cell is the skeleton and engine for all the organisms. There are unicellular and multicellular INTRODUCTION organisms who have cell...

 CELL CYCLE AND CELL DIVISION 1. Introduction Topics Discussed The cell is the skeleton and engine for all the organisms. There are unicellular and multicellular INTRODUCTION organisms who have cells in their body. The life to CELL CYCLE exist and continue the cell needs to divide like any MITOSIS other organism. Thus, the cell division is also a fundamental and essential function. Cell division is a MEIOSIS long and complex process which involves several AMITOSIS steps, common in all the organisms. As unicellular organisms have a single cell which divides in order to increase the population and also continue the species on the planet. Multicellular organisms have several cells in their body which are specialised to perform several functions. Thus, the cell division here ensures that the body is growing, developing, its repair, maintenance and also in the reproduction of the organism. There are various methods for a cell to divide namely Mitosis, Meiosis and Amitosis. There are several processes occurring before the actual cell divides into two. In this chapter we will discuss the processes of the division.  Cell Cycle and Cell Division Objectives of this Chapter At the end of this chapter you will be able to: ⚫ Write about the phases that occur in a cell. ⚫ Arrange the phases of a cell division in order. ⚫ Distinguish between mitosis and meiosis. 2. Cell Reproduction or Cell Cycle ⚫ The cell cycle involves three major processes – Cell growth (time required by a cell for synthesis and duplication of various components of the cell), DNA replication (time when the DNA replicates) and cell division (an adult mature cell finally divides into two daughter cells). ⚫ A typical eukaryotic cell cycle is represented with a human cell in various culture methods. These cells divide once in approximately every 24 hours. ⚫ Yeast cell has ability to finish the cell cycle in about 90 minutes. 2.1 Cell Cycle and Its Phase The cell cycle is divided into two basic phases. Howard and Pelc classified interphase into three sub stages. ⚫ Interphase ⚫ M-phase (Mitosis phase) 2.1.1 Interphase ⚫ The preparatory phase, resting phase, beginning phase and also a phase involving great metabolic activity. The intermediate stage between the two consecutive cell divisions where no cell division or even chromosomes division takes place. However, the nucleus and cytoplasm are metabolically and synthetically very active in order to get prepared for the division. The length of this phase is 90% - 95% of the total cell cycle. The series of events occurring in the cell in this particular phase are: replication of DNA, synthesis of nuclear histones, division of centrioles to form a new pair of centrioles, synthesis of energy rich compounds, RNA and proteins. The nuclear envelope remains intact, however, the nucleolus show genetic DNA as long, coiled, indistinctly visible chromatin fibres in the chromosomes. Also, there are rRNA and ribosomal proteins accumulated in nucleolus which greatly increases its size. Interphase is further divided into three phases: (i) G1-phase (ii) S or Synthesis Phase (iii) G2-Phase Cell Cycle and Cell Division Figure: Cell cycle involved in cell growth and division (i) G1-Phase It occurs at the end of a mitotic division (pro-mitotic gap phase). The initiation of DNA replication is major function. Following biochemical changes are common during this sub-stage. ⚫ The cell grows until its maximum size as the normal metabolic activity occurs for the DNA replication preparation, and DNA contents of the cell remains unchanged. ⚫ The new proteins are translation and RNA: rRNA, tRNA and mRNA transcription occurs during this phase. ⚫ Also Nucleotides, amino acids and ATPs are formed. ⚫ The most variable phase which differs in time affecting the cell division duration for each cell. G1 under certain stimuli can be terminated. Once G1 is completed in a cell and ‘S’ phase has started with the replication of DNA, it cannot be terminated. ⚫ There are cells which do not exhibit division usually in animal adults (e.g., heart cells) and also some which divide occasionally, as and when required to replace the lost or injured cells. Once replacement is complete, these cells stop further division and exit G1 phase. Then they enter an inactive stage called as the quiescent stage (G0) in the cell cycle. The cells are metabolically active, however, do not proliferate till the requirement. Hence, this phase of G0 can be temporary or permanent in the organism. Antephase, the end of G1 phase where the cell will divide in all the conditions even under stress conditions.  Cell Cycle and Cell Division (ii) S or Synthesis Phase ⚫ The synthesis or replication of DNA on the template or the existing DNA takes place. ⚫ The amount of DNA in a cell doubles (means the cell has twice the normal set of genes). However, the chromosome number remains the same (Ploidy level remains same). Assume: the initial amount of DNA as 2C, then the DNA amount increases to 4C, and the cell has 2n number of chromosomes at G1, which remains the same even after S-phase. ⚫ The replication occurs inside the nucleus along with centriole doubling in the cytoplasm. ⚫ Histone proteins are also synthesised in S-phase. This phase is called as invisible phase of the cell cycle as the replicated chromosomes are invisible. (iii) G2 Phase The phase just before the mitosis (pre-mitotic gap phase). ⚫ The cytoplasmic organelles multiply like mitochondria, chloroplast and Golgi complex. ⚫ Transcription of RNA and then translation protein continues. Spindle tubulin synthesis and aster formation starts. ⚫ A cell contains double the number (4C) of DNA present in the original diploid (2N) cell. ⚫ The cell is now prepared to enter into “M” or Mitotic phase. ⚫ The main part is the synthesis of some protein kinases used in the regulation of cell division. Kinases regulating the cell cycle are called as Cdks (cyclin dependant kinases) because they get activated after combination with the key protein called as cyclin. ⚫ The kinase enzyme along with cyclin moves the cell cycle in forward direction. S-kinase is capable of the DNA replication initiation after it combines with S-cyclin. After some time S-cyclin gets destroyed and S-kinase loses its activeness. Cell cycle in the meristem cells are with a special protein “Cyclin and Cdks” (discovered by Nurse, T. Hunt & Hartmann 2001 during the experiment on yeast cell). The cyclin protein triggers the DNA replication. 2.1.2 M-Phase The phase when the actual cell division or mitosis is initiated. The steps involve nuclear division, the separation of daughter chromosomes (Karyokinesis) and ends in the division of cytoplasm (cytokinesis). The 24 hour is the average duration of cell cycle in a human cell, where the cell division, i.e., M-phase lasts for about an hour. Cell Cycle and Cell Division 3. Mitosis ⚫ Mitosis produce genetically identical cells. The chromosomes undergo division and replicate to form duplicates which are similar to mother cell chromosome number (equational division). ⚫ The division is also called as somatic cell division or equational division or indirect division. ⚫ Mitosis was coined by Fleming in 1882. ⚫ Establishment: Strasburger observed mitosis in plants. While Boveri and Fleming observed the same in animals. ⚫ Duration: Dependent on the type of the cell involved and its species. It takes 30 minutes to 3 hours. The various factors affecting the duration are type of the tissue, its location, temperature and species of the organism. The actual cell division is for one hour from the 24 hour average duration. ⚫ Occurrence: A common division method for both the somatic or body cells and the germ cells in the sex organs. There are phases and specific location where it is common and a regular method. Plant meristematic tissues (root and shoot tips) and animal skin, bone marrow, even the embryonic developmental stages have the mitotic division. ⚫ Cause of mitosis: Kern plasm theory: Hertwig proposed kern plasm theory. According to this theory mitosis occur due to disturbance in Karyoplasmic index (KI) or nucleocytoplasmic ratio of cell. Vn = Volume of nucleus Vc = Volume of cell Vc-Vn = Volume of cytoplasm Karyoplasmic Index (KI) of small cell is high as they have less cytoplasm. Nucleus efficiently controls the activity of cytoplasm in small cells, so these cells are metabolically more active  Cell Cycle and Cell Division During cell growth, cytoplasm increases, thus K.I. decreases. In a large cell, nucleus fail to control the activity of cytoplasm. To attain the control of nucleus on metabolism a large cell divides into two cells. ○ Surface-volume Ratio: A cell draws all the materials needed for its maintenance and growth from its surface. When a cell grows in size its volumes increases more than its surface. So a stage will reach when the surface area becomes insufficient to draw the material. At such critical stage, division of cell started. 3.1 Phases of Mitosis The phases of mitosis are as follows: Interphase (as described earlier). DID YOU KNOW ⚫ The factors responsible for the cell division to occur or not occur are as: ○ Surface area to the volume ratio: in order to undergo division a cell should have low surface area to volume ratio. ○ Karyoplasmic index: Also a cell should have low karyoplasmic index (explained later in the chapter). ⚫ Ideal examples for the mitosis study is onion root tips or other meristematic tissues. ⚫ Mitogens are mitosis inducing substances. E.g., Auxin, Cytokinin, Gibberellin, Insulin etc. ? ⚫ Mitosis in animal cell is called as Amphiastral division as it has the spindle formation associated with 2 asters at each pole. ⚫ Mitosis in plant cells is called as Anastral division where there are no aster and no centriole. ⚫ The mitosis when occurs in a cell which has lost its nuclear membrane and is extra nuclear, it is called as Eumitosis. ⚫ The mitosis is intranuclear where it occurs in the nuclear membrane while it is still in the cell, it is called as Premitosis. ⚫ The centrioles when form the spindle complex in the cell it is called as centric division. Division phase or M-phase or mitotic phase (duration 1hr) is the most dramatic period of the cell cycle. Karyokinesis – Division of nucleus; and Cytokinesis – Division of cytoplasm. Cell Cycle and Cell Division Figure Different stages of mitosis  Cell Cycle and Cell Division Figure: Chromosome showing spindle formation 3.1.1 Karyokinesis Division of nucleus occurs by sequential changes (Indirect division) Karyokinesis has 4 stages: (i) Prophase (longest stage) ⚫ Chromatin threads get condensed to form the chromosomes. ⚫ Centrioles get aligned towards the opposite poles. ⚫ Astral ray formation from the proteins gelatinised around the centrioles (initiation of the assembly of mitotic spindle). ⚫ Cells do not show Golgi complexes, ER, Nucleolus and nuclear membrane at the end of the prophase. (ii) Metaphase ⚫ The nuclear envelope is completely disintegrated which highlights the start of the second phase in mitosis. The chromosomes spread throughout the cytoplasm. Spindle fibres attach to the chromosomes at their kinetochores. ⚫ The condensation of chromosomes is complete. This is the stage where morphology of chromosomes is easily visible. ⚫ The chromosome is compiled in two sister chromatids, held together with the centromere. ⚫ Each chromosome splits as per length upto the centromere (division of matrix of chromosome). Thus, replicated chromatids are clearly visible at metaphase stage. ⚫ Chromosomes spilt up and arrange themselves on the equator to form metaphase plate (equatorial plate). Cell Cycle and Cell Division ⚫ Spindle fibres are microtubules. Chromosomal fibres, (discontinuous and run from pole to centromere) and supporting fibres, (continuous and run from pole to pole), arrange in a cell. ⚫ The centromere lies at the equator with arms facing the poles. Figure: Metaphase stage (iii) Anaphase (smallest stage) ⚫ The early anaphase have inter zonal fibres appearing at the equator. ⚫ Chromosome centromere splits lengthwise (division of centromere). ⚫ Chromosomes double inside a cell during mitotic anaphase. Every chromosome has one chromatid. ⚫ Expansion of Inter zonal fibres and the chromosomes are pushed towards the opposite poles (pushing) ⚫ Contraction of chromosomal fibres such that they pull them towards opposite poles (pulling) Figure: Anaphase stage (iv) Telophase (reverse of prophase) ⚫ Nuclear membrane, Nucleolus, Golgi complex and ER now surround each of the chromosomal pole. ⚫ The chromatin net is formed after the chromosomes decondense. Chromosomes lose their individuality which means the individual chromosomes are not present.  Cell Cycle and Cell Division Figure :A. Early telophase and B. Late telophase stage 3.1.2 Cytokinesis ⚫ Cytokinesis is initiated in late the anaphase. It is different for plants and animals. (i) Cytokinesis in animals ⚫ It occurs through constriction and furrow formation in the cell membrane. ⚫ A mid-body equator is formed when the microtubules arrange in the middle while the microfilaments arrange in the peripheral ring just below the plasma membrane. ⚫ The cell organelles arrange themselves at either side of the equator. ⚫ The contraction occurs as the attraction occurs between mid-body and peripheral ring, forming a furrow from the outside of the cell to inside. Figure: Cell membrane division in animals ⚫ The furrow formed deepens continuously and finally the cell divides into two daughter cells. ⚫ The cytokinesis in animal cell occurs in the centripetal order. Cell Cycle and Cell Division (ii) Cytokinesis in plants ⚫ The cell plate formation takes place because the constriction or even furrow is not possible as the cell wall is rigid. Figure: Cell division in plants ⚫ Many Golgi vesicles and spindle microtubules arrange themselves on equator and the cell has a Phragmoplast. It may also have the deposits of fragments of ER. Golgi vesicles membranes fuse and form a plate like structure which is called as the cell plate. Golgi vesicles then secret pectates of calcium and magnesium. The cell plate modifies into the middle lamella. The cytokinesis of plant cells occur in the centrifugal order (cell plate formation is from centre to periphery). 3.2 Significance of Mitosis ⚫ Mitosis: The equational division is a common division method for the diploid cells only. However, some lower plants and social insects which have haploid cells, also use mitosis for division. The significance of this division is essential to understand in the life of an organism. ⚫ Mitosis results in the production of diploid daughter cells which have identical genetic chromosome number. The multicellular organisms grow due to the mitosis. ⚫ Cell growth often results in disturbing the usual ratio of the nucleus and the cytoplasm. Thus, the cell divides and restores the nucleo-cytoplasmic ratio. ⚫ A very significant contribution is that a cell is repaired. Best examples are the cells of the upper epidermis layer, cells of the gut lining, and blood cells being replaced constantly.  Cell Cycle and Cell Division KNOWLEDGE BUILDER Mitotic Poisons: All the substances or chemicals which affect the mitotic process in a cell or prevent the cells completely from dividing normally are called as mitotic poisons. The various mitotic poisons are: ⚫ Enzyme ribonuclease, Azide and cyanide acts as a poison during prophase. ⚫ Mustard gas reaching a cell results in the agglutination of the chromosomes. ⚫ Chalones are small peptides or glycoproteins present in the extracellular fluid also inhibit mitosis. ⚫ The alkaloid colchicine targets and inhibits the formation of mitotic spindle (inhibits polymerization of microtubules) and freezes the cell in the metaphase. Though chromosomes and DNA replicate they remain intact in the same cell. The nucleus division dies not occur. This increases the chromosome sets in a cell. This process leads to endopolyploidy or endomitosis in which nucleus contains multiple sets of chromosomes, more than the normal two sets in a normal diploid cell. Such cells are called as polyploidy cells. ⚫ X-rays induce uncontrolled mitosis as they energize the cells and thus cause breakage of chromosomes. Abnormal Mitosis: ⚫ Intranuclear mitosis (pre-mitosis): In Amoeba, Yeast, fungi and many algae, during the mitotic division, the nuclear envelope fails to degenerate. Spindle formation is intranuclear. ⚫ Dino mitosis: Dinoflagellates possess condensed chromosomes even in non-dividing nuclei. Nuclear envelope does not degenerate. Division of chromosomes occur when the nucleus develop special channels. ⚫ Free Nuclear Division: Sometimes, there are repeated mitosis without the subsequent cytokinesis in a cell which results in multinucleated conditions, e.g., Rhizopus, Vaucheria, Slime moulds, etc. 4. Meiosis ⚫ Meiosis is a method where the division produces genetically different type of cells. All the four daughter cells produced with meiosis have genetic differences among each other and also are different from the mother cell. Gametogenesis the formation of gametes is a common factor for meiosis to occur. Cell Cycle and Cell Division 4.1 Phases of Meiosis There are two different phases in the division of cell: ⚫ Meiosis I: Heterotypic division or reduction division. It leads to reduction in chromosome number to half in daughter cells. Division of chromosome does not occurs in meiosis-I, only segregation of homologous chromosomes takes place. ⚫ Meiosis II: Homotypic division or equational division. It does not lead to any change in chromosome number. Division of nucleus occurs twice, however, the DNA replication and chromosome division occurs only once. Flowchart: Phases of Meiosis 4.1.1 Stages of Meiosis I (i) Prophase - I: The longest and most complex stage of the meiosis. Prophase I is further divided in five sub stages as: (a) Leptotene – Chromatin threads are condensed so that they form chromosomes which are longest and thinnest fibers. There are bead like structures present on it called as chromomeres. All the chromosomes move towards centrioles in nucleus, so group of chromosomes in nucleus appears like a bouquet in animal cell. (Bouquet stage).  Cell Cycle and Cell Division Figure: Leptotene stage (b) Zygotene or Synaptotene – There is pairing of homologous chromosomes (Synapsis). The pairs of homologous chromosomes which are formed here, are called as Bivalents or Tetrads, and are clearly identified in the next stage. A structure is developed in between the homologous chromosomes, is called as synaptonemal complex. It has three thick lines made up of DNA and proteins. The complete set helps in pairing of the DNA. Figure: Zygotene stage (c) Pachytene (thick thread) – There is increased attraction which causes homologous chromosomes to coil tightly around each other. Both the chromatids in the chromosome are clear and distinct and now the pair or bivalent is found as a tetrad. Both the chromosome chromatids are called as sister chromatids. Non-sister chromatids present in the bivalent develop into recombination nodules and exchange their parts called as the crossing over. This is an enzyme-mediated process and the enzyme is recombinase. Figure: Pachytene stage Cell Cycle and Cell Division (d) Diplotene – The diplotene starts with the dissolution of the synaptonemal complex. There is also the tendency in the bivalent recombined homologous chromosomes to separate from each other while still joint at the cross-overs. These X-shaped structures formed are called as chiasmata. The diplotene can last for months or years, in some vertebral oocytes which is called as dictyotene. Figure: Diplotene stage (e) Diakinesis – The meiotic prophase I ends in diakinesis. There is markable terminalisation of the chiasmata. The chromosomes gets fully condensed and then the meiotic spindle assembles to prepare the homologous chromosomes which separate. When diakinesis ends, the nucleolus disappears and the nuclear envelope breaks down. Diakinesis ends and metaphase starts. Figure: Diakinesis stage (ii) Metaphase I: ⚫ Bivalents form metaphase plate after arranging on the equator of cell such that the centromeres face the poles while arms face the equator. ⚫ Spindle fibres now attach to the pair of homologous chromosomes. ⚫ There are in all 3 types of spindle fibres in the cell: ○ Chromosomal / Kinetochore Spindle fibres ○ Supporting / Continuous Spindle fibres ○ Inter zonal Spindle fibres.  Cell Cycle and Cell Division Figure: Metaphase I stage (iii) Anaphase I: ⚫ There is contraction of chromosomal fibres and expansion of inter zonal fibres. The homologous chromosomes move towards the opposite poles after they segregate from each other. ⚫ Anaphase I has segregation or disjunction of the homologous chromosomes. There is no division of centromere. Figure: Anaphase I stage (iv) Telophase I: ⚫ The nuclear membrane and nucleolus reappear. ⚫ This is followed by the cytoplasm division or the cytokinesis and two daughter cells together are called as diad of cells. The chromosomes in some situations undergo some dispersion, and are thus fail to reach the extremely extended state of the interphase nucleus. Cell Cycle and Cell Division Figure: Telophase I stage A. and B. are daughter cells ⚫ The connecting stage of the two meiotic divisions is called as interkinesis which is short in duration. DNA does not replicate in this stage. Interkinesis ends with the start of prophase II, which is simpler than prophase I. Figure All stages involved in Meiosis  Cell Cycle and Cell Division 4.1.2 Stages of Meiosis – II (i) Prophase II: ⚫ Meiosis II is an intermediate step which starts immediately after cytokinesis, and before the chromosomes have elongated fully. Meiosis II is similar to a normal mitosis, in contrast to meiosis I. The nuclear membrane disappears and chromosomes are compact again in the end of this stage. (ii) Metaphase II: ⚫ The chromosomes get aligned at the equator while at the opposite poles the spindle microtubules are in close contact with the kinetochores of the sister chromatids. (iii) Anaphase II: ⚫ The simultaneous splitting of the chromosome centromere occurs (which was holding the sister chromatids together), which moves the chromosomes toward the opposite poles of the cell. (iv) Telophase II: Figure: Different stages in Meiosis II ⚫ The two sets of chromosomes are again enclosed in a nuclear envelope and cytokinesis begins. There is formation of tetrads (four haploid daughter cells). 4.2 Significance of Meiosis ⚫ Meiosis is the division in which specific chromosome number in each species is conserved. This is achieved in sexually reproducing organisms across several generations, even though there is reduction of chromosome number by half in the whole process. Cell Cycle and Cell Division ⚫ The genetic variation increases in the population of organisms over various generations. Evolution is due to variations which is a very important factor that is progressive with time. DID YOU KNOW ⚫ Onion buds (Sambhar onions) are common Meiotic study cells. ⚫ Van Beneden first demonstrated Meiosis and Winiwarter described it. ⚫ Gametic meiosis in the gametic cells is also called as terminal meiosis. ⚫ Zygotic meiosis when the zygote formation is initiated is also called as initial meiosis. ? ⚫ ⚫ Sporogenic meiosis is also called as intermediate meiosis. Cytokinesis: Cytokinesis can be of two types, successive and simultaneous. Cytokinesis occurs after every nuclear division in successive division. The four cells formed after the successive cytokinesis can be arranged linearly or isobilateral in tetrads. When cytokinesis occurs at the end of both the divisions it is the simultaneous division. The nuclei arrangement is in the form of a tetrahedron. KNOWLEDGE BUILDER The best theory to explain crossing over is Darlington’s theory of breakage and union: ⚫ The enzyme endonuclease develops breaks called as nicking. ⚫ The gap formation in the nicks is due to exonuclease. ⚫ The chromatid segments separate in the gaps due to the U-protein or helicase enzyme. ⚫ Re-annealing (rejoining) is a result from the R-protein or Re-annealing protein. The newly formed chromosomes, are different from the parent cell chromosomes. The formation of new characters (recombinants) and ultimately variations are the obvious results in the population which result in the evolution.  Cell Cycle and Cell Division TRY IT YOURSELF 1. If a pollen grain contains 30 chromosomes and 50 Pg DNA, then what will be the number of chromosomes and DNA amount in microspore mother cell in G2-phase, meiosis-I products and meiosis II products respectively? 2. Fill in the blanks: a. Darkly stained bead like structures called appear along the entire length of chromatin fibre in stage of meiosis. b. Number of bivalents formed in Ophioglossum (Adder’s tongue fern) meiocyte is. c. Desynapsis phase is. 3. Intrameiotic interphase is characterised by (A) Genes duplication (B) replication of DNA (C) Centrioles duplication in animals (D) Disappearance of nucleolus 5. Amitosis ⚫ The most primitive type of cell division. Condensation of chromosomes not occurs in amitosis. Chromosomes are not visible during division. The process of division does not have any recognizable chromosomes in the cell. ⚫ Amitosis is without the spindle formation. Nucleus division is direct i.e. without sequential changes (prophase, metaphase, anaphase and telophase). ⚫ The division of cytoplasm and nucleus is simultaneously through the constriction. ⚫ The division may be equal or unequal in the chromosome number. ⚫ Amitosis is the fastest cell division method which can be completed in just 20 to 30 minutes. Amitosis is the cell division method of prokaryotes. However, exceptionally it also occurs in some eukaryotes, e.g. in budding Yeast. Table 12.1: Some formulae related to cell division Formulae Chart 1. Number of mitotic divisions for the formation of n number of cells. n–1 Example: For getting 100 cells 99 mitotic divisions are required 2. Number of generations (n) of mitosis for producing ‘x’ cells. x = 2n 3. Number of meiosis for the formation of ‘n’ seeds/grains/fruits. n n 4 Cell Cycle and Cell Division